If you've held the stock for more at least one year,
you are correct, and you will be able to deduct the Fair Market Value of the
donation.
However, if you have held the
stock for less than one year, it is short term capital gains property, and you
can only claim your basis as a deduction.
As long as you've held the property for one year, the
amount that you enter for the basis will not affect your deduction.
For more information - Publication 526 (2017),
Charitable Contributions: https://www.irs.gov/publications/p526
